Carbon stars are evolved cool giants
(and some time dwarf) with circumstellar shells or clouds of carbon
dust material. The typical temperature surface range from 2000K
and 3000K. The apparent color of this stars is deep red and all
are irregular or semi-regular variables.

The C2 Swan bands dominate the spectral distribution
(at wavelength of 4380A, 4738A, 5165A, 5635A, 6122A). Also visible
are the absorption C3 bands (4056A), CN (i.e. cyanogen at 3880A,
4217A, 7699A), CH and SiC2 bands, and often strong Na line at 5893A.
One of the classification scheme of spectra is like C2,3. The first
number indicates decreasing temperature from 0 to 9 (2 for the example).
The second number indicates strength of the carbon bands from 1
to 5 (3 for the example). Carbon stars are also classified into
N and R series, where N stars are older stars. An N9 star is redder
and cooler than a N1 star.